1. Field of the Invention
The present invention relates to optical devices such as integrated semiconductor devices for repeating a signal, integrated light branching-combining devices and semiconductor optical amplifiers. The present invention also relates to optical communication networks or systems using the optical devices.
2. Description of Related Background Art
In recent years, there has been an increasing demand for optical devices used in optical local area networks (optical LAN) in the field of optical communications. A conventional optical LAN is shown in FIG. 1. In the optical LAN, a transmitted signal is taken into respective terminals, and the signal is logically processed in each terminal and re-transmitted therefrom. In such a system, optical nodes 12-1.about.12-4 need to have a structure as shown in FIG. 2. FIG. 2 illustrates the optical node 12-2 as an example. In this structure of the node, there are provided opto-electric (O/E) converters 14-1 and 14-2 for respectively converting a signal transmitted from each of transmission lines 11-1 and 13-1 to an electric signal, a control circuit 16 for processing an electric signal, electro-optic (E/O) converters 15-1 and 15-2 for converting the electrically processed signal to a light signal to be re-transmitted to transmission lines 11-2 and 13-2, and the like.
Generally, communication is performed by the above means in a network as shown in FIG. 1. There may be a case where communication is interrupted because of trouble in the network. Therefore, in order to improve the reliability of the network, the optical nodes 12-1.about.12-4 need to have a function for coping with trouble when it occurs. A conventional trouble countermeasure method used in the optical communication network shown in FIG. 1 will be described.
Possible types of trouble or failures are breakdown of the optical fibers 11 and 13, failure of the optical nodes 12 to function and so forth. Trouble in the communication network may include a case where the transmission line or the optical fiber is intentionally disconnected at the time of addition of optical nodes or the like, and a case where an electric source in the optical node is switched off. When an optical node 12 is out of order, the network is divided into two portions at the boundary of the optical node in trouble.
A countermeasure for such trouble will be described using the system shown in FIG. 1. If the transmission line is a single line in an ordinary optical communication network, the countermeasure of the division of the network cannot be used. Therefore, in order to solve such a hardship, two series or sets of transmission lines, O/E converters and E/O converters are provided for respective optical nodes 12. One system is used as a reserve. In FIG. 1, reference numerals 11-1.about.11-4 designate optical fibers to be used as an ordinary transmission line, reference numerals 12-1.about.12-4 designate optical nodes and reference numerals 13-1.about.13-4 designate optical fibers to be used as reserve transmission lines when trouble occurs. Each of the optical nodes 12-1.about.12-4 has the structure shown in FIG. 2, and has communication functions for the transmission lines 11 and 13.
First, an operation that takes place when trouble occurs in the optical node 12 will be described.
If trouble occurs in optical node 12-2, the trouble is detected when a communication is routed through the optical node 12-2. Upon detection of trouble, the optical node 12-1 notifies other optical nodes 12-3 and 12-4 of the fact that trouble has occurred in the optical node 12-2 on the ordinary transmission line, by utilizing the reserve O/E and E/O converters in the optical node 12-1 and by using the reserve transmission system. Upon this notice, the other optical nodes 12-3 and 12-4 change from ordinary communication to a communication that is performed by the reserve O/E and E/O converters and the reserve transmission series. Thus, communication after trouble has occurred can be secured. However, if trouble occurs in the entire optical node or in both of the two series, then it becomes impossible to route a communication through the optical node in trouble in the network.
The operation that occurs when an optical fiber is broken down will be described next. If the optical fiber 11-2 is broken down, this disconnection is detected when a communication passing through the optical fiber 11-2 (e.g., communication between the optical nodes 12-1 and 12-3) is performed. Upon this detection, the optical node 12-1 notifies other optical nodes 12-3 and 12-4 of the fact that trouble has occurred on the transmission line usually used, similar to the above, by changing O/E and E/O converters in the optical node 12-1 and using the reserve transmission series. Upon this notice, the other optical nodes 12-3 and 12-4 change from ordinary communication to a communication that is performed by the reserve O/E and E/O converters and the reserve transmission series. Thus, communication after the breakdown can be also secured. However, as described above, the following problems exist in the countermeasure for trouble in the active optical LAN as shown in FIG. 1.
(1) All communication function will be lost if trouble occurs in the entire optical node; PA1 (2) A reserve communication series is needed; and PA1 (3) All optical nodes are required to secure a state in which reproduction and relay (i.e., repeating) is possible. PA1 (1) A failure-safety function is not established; PA1 (2) Compensation for insertion loss is disregarded; PA1 (3) Cost performance and extensibility are low; and PA1 (4) A high fabrication precision is required to obtain a desired branching ratio.
Those facts mean that the network always needs two sets of optical nodes and two sets of transmission lines, leading to a costly system. Furthermore, the problem (3) becomes serious, when the network is extended over a wide range. It is also a serious burden to warrant the operation of the entire network.
Further, in optical communication networks such as optical LAN devices, such as light branching-combining devices and demultiplexing or multiplexing devices, there are key devices for determining the specifications of a system.
An example of the light branching-combining device is disclosed in Erman et al. "Mach-Zehnder Modulators and Optical Switches on III-V Semiconductors", Journal of Lightwave Technology, vol. 6, No. 6, pp. 837-846, 1988. FIG. 3 is a plan view thereof, and FIG. 4 is a cross-sectional view of a light waveguide which forms input and output ports. In FIG. 3, reference numeral 21 designates an input port, and reference numerals 22 and 23 designate output ports. Light incident on the input port 21 is branched into two portions propagated through the output ports 22 and 23, by a beam splitter 24. In FIG. 4, reference numeral 33 designates a core layer, and the effective refractive index in a transverse direction is defined by a groove 37 formed in a cladding layer 32 layered on an n.sup.+ -GaAs substrate 31 so that a transverse mode of the propagated light can be stabilized. Reference numerals 34, 35 and 36 respectively designate an n-type Al.sub.0.5 Ga.sub.0.5 As layer, an n-type Al.sub.0.1 Ga.sub.0.9 As layer and a p-type GaAs layer formed on the core layer 33. The beam splitter 24 is composed of a perpendicular groove which horizontally extends at an angle of 45.degree. relative to the light propagation direction and whose depth is ended halfway of the core layer 33. Side faces of the perpendicular groove act as a total reflection mirror. As a result, the branching of the light incident from the port 21 into the port 22 is performed by the total reflection mirror, while the branching thereof toward the port 23 is achieved by a wave-front splitting with respect to the depth direction.
The structure as shown in FIGS. 3 and 4, however, has several drawbacks as follows:
It is highly desirable that devices disposed ln optical communication networks have a function to automatically overcome trouble in the network (so-called failure-safety function) so that the network is not influenced thereby even when trouble occurs in the device itself, a terminal or a terminal unit connected to the device. However, this function is impossible to achieve in the above-discussed device. Although other devices, such as an ordinary optical switch, can be disposed in order to obtain the failure-safety function and the function to vary the branching ratio, those devices have to be disposed externally. Therefore, the advantageous effects of integration and extensibility will be lost, and size and cost will increase. Further, the device structure inevitably becomes complicated because other devices, such as an optical amplifier, are needed, for example, to compensate for light loss due to the light splitting.
Furthermore, it is advantageous for the branching ratio of the light branching-combining device that is disposed in the optical network to be variable in order to achieve flexibility and expansibility in the network.
Further, in optical communications such as an optical LAN, the importance of devices such as optical amplifiers has been increasing in recent years. Optical amplifiers can be roughly classified into fiber amplifiers and semiconductor amplifiers, and they are respectively used for different purposes.
The semiconductor amplifier or laser diode amplifier (LDA) is advantageous in that its structure is suited to integration since the LDA has the same structure as the semiconductor laser. An example, in which devices such as the LDA are integrated, is disclosed in K. Y. Liou et al. "Monolithically Integrated GalnAsP/InP Optical Amplifier with Low Loss Y-branching Waveguides and Monitoring Photodetector", Lecture No. CDF7, Conference on Laser and Electrooptics, 1990. FIG. 5 shows such an example.
In FIG. 5, optical amplifier portion 41, Y-branching portion 42 and PIN photodetector portion 43 are integrated on an InP substrate 40. In its layer structure, a light guide layer (its band gap in wavelength: 1.1 .mu.m) is formed under an active layer (its band gap in wavelength: 1.3 .mu.m), and the active layer is common to the optical amplifier portion 41 and the photodetector portion 43. In the Y-branching portion 42 the active layer is removed, and antireflection coatings are provided on facets of the optical amplifier portion 41 and photodetector portion 43. This structure features a very small waveguiding loss and coupling loss in the respective waveguides and the branching of a light wave into two portions by the Y-branching portion 42 where the two portions can be respectively monitored.
The above described amplifier, however, has the following disadvantage. Since the light signal necessarily passes through LDA, the light signal will be greatly disrupted if trouble occurs in the LDA. The amount of carrier injection in the LDA is larger than that of a laser diode, so that the probability of internal degradation of the LDA is considerably greater than that of the laser diode. Therefore, the LDA needs to have a failure-safety function in case the degradation occurs. To achieve this purpose, a method can be considered in which ordinary optical switches are disposed in front of and at the rear of the LDA. For example, Japanese Patent Publication No. 4-64044 discloses a method wherein optical components such as lens and prism are externally disposed. According to this method, however, since external disposition needs to be adopted, size and cost of the device necessarily increase and the merit of integration is lost. Thus, the advantages particular to the LDA will be lost.